Popular well established display technologies such as liquid crystal displays as well as emerging display technologies such as organic light emitting diode displays are active-matrix displays. An active matrix display comprises a matrix of thin film transistors. Polycrystalline and more recently amorphous silicon has traditionally been used as the semi-conductor channel layer of choice in active matrix displays. The main drawback of amorphous silicon however is its low charge carrier mobility (around 1 cm2/V·s for electrons) which limits device performances. Alternatives are now being proposed belonging to the so-called oxide semi-conductor family. For instance, In (indium) Ga (gallium) Zn (zinc) oxides have been produced with charge mobility in the range of from 10 to 40 cm2/V·s. Furthermore, In Ga Zn oxides allow for homogeneous large area deposition. This makes In Ga Zn oxides the favoured oxide semi-conductor for active matrix displays of the next generation. A major drawback of In Ga Zn oxides is the fact that In is both toxic and in limited supply on earth. In is expected to be the first element where resource limitations will limit the growth of future display technologies. There is therefore a need in the art for viable Indium-free alternatives to In Ga Zn oxides. Zn Sn oxides and Zn Sn Ga oxides have been proposed as alternatives but their manufacture in acceptable quality is very challenging.
The paper “The influence of mechanical activation on zinc stannate spinel formation” by N. Nikolić et al. in the Journal of the European Ceramic Society 21 (2001) studies the formation of zinc stannate ceramics during treatments of compacts obtained from ZnO and SnO2 powder mixtures mechanically activated in a high energy mill, and explains that zinc stannate is favourably formed by a solid state reaction during sintering with increasing time duration of high energy milling.
US20070215456A1 discloses a sintering process for the manufacture of Zn Sn oxide targets and of a Ga Zn Sn oxide sputtering target. The process involves long milling (18 hours) of the raw material, sintering in an oxygen containing atmosphere for longer than 15 hours and subsequent reduction processing by heating under non-oxidative atmosphere for longer than 7 hours to make a single sintered body. US20070215456A1 illustrates by means of examples that deviation from the preferred procedure can lead to high arcing rate of the target, and insufficient strength of the sintered body. Insufficient strength can be caused by not containing zinc stannate compound phase (or low amounts of this phase) in the sintered body. US20070215456A1 makes no mention of the length of the obtained sputter target (single sintered body) but proposes that, if necessary, several pieces of the sintered bodies may be arranged in a divided form to make a target with large area. The resistivity of the Zn Sn oxide targets was ranging from 2.3 kΩ·cm to 4.7 kΩ·cm and the resistivity of the Zn Sn Ga oxides target was 0.11 kΩ·cm. US20070215456A1 indicates that these sputtering targets could be used at a DC charge power density of 5.513 W/cm2. Although these numbers compare favourably with the rest of the prior art, there remains room for much improvement. Furthermore, the sintering method at present used for the manufacture of these sputtering targets is very tedious and labour-intensive.
There is therefore a need in the art for new Indium-free alternatives to In Ga Zn oxides sputtering targets, especially for large area coatings.